The invention relates generally to exciter circuits for ignition systems used with internal combustion engines. More particularly, the invention relates to exciter circuits that utilize solid-state switches such as, for example, thyristors, as control devices for exciter circuit oscillatory discharge control.
A conventional ignition system for an internal combustion engine, such as, for example, a gas turbine aircraft engine, includes a charging circuit, a storage capacitor, a discharge circuit and at least one igniter plug located in the combustion chamber. The discharge circuit includes a switching device connected in series between the capacitor and the plug. For many years, such ignition systems have used spark gaps as the switching device to isolate the storage capacitor from the plug. When the voltage on the capacitor reaches the spark gap breakover voltage, the capacitor discharges through the plug and a spark is produced.
More recently, turbine engine and aircraft manufacturers have become interested in replacing the spark gap with a solid-state switch, such as an SCR or thyristor. This is due, in part, because a solid state switch typically operates longer than a spark gap tube which may exhibit electrode erosion. Also, solid state switches are produced in large volume making them less expensive than spark gaps which are individually crafted in small quantities. Furthermore, the storage capacitor's voltage at discharge remains essentially constant over the life time of the solid state switch, but can change significantly during the life of the spark gap due to electrode erosion.
In order to produce high peak powers at the igniter plug tip, high di/dt levels are generated with the exciter circuit. These high current transition rates create voltage and current reversals due to stray inductances that are present within the discharge circuit. When spark gap tubes are used as the switching device these voltage and current reversals are tolerable. However, solid state switches, such as thyristors, can be damaged by such reverse voltages. Consequently, exciter circuits employing the use of solid state switches typically include protective circuits to prevent the reverse voltages or to lessen their effect on the switches.
A common technique for preventing reverse voltages is to place a free wheeling diode on the discharge side of the switches to force a unidirectional discharge current through the igniter.
However, there are engine applications for which the use of an oscillatory discharge is required by the customer or end user. In such cases, the free wheeling diode cannot be used to protect the solid state switches. It is also necessary that the thyristor switches be able to conduct current every other cycle during the oscillatory discharge. If a switch turns off during a reverse current portion of the discharge, the switch must be turned back on for the next forward current portion of the discharge cycle.
An oscillatory discharge exciter design using an SCR thyristor is illustrated in U.K. Patent No. 962,417. This design includes the use of an SCR as the switching device and a reverse parallel diode to conduct the reverse discharge current relative to the direction of current flow through the switch. This simple design, however, is not suitable in many applications because the SCR could recover and block forward current flow during the negative current half-cycles.
The objective exists, therefore, for an oscillatory discharge exciter circuit that uses solid state switches and that can assure that the switching devices are in conduction for the forward current discharge portions of each oscillatory discharge cycle.